The crude petroleum oil charge entering a petroleum refinery contains a number of impurities harmful to the efficient operation of the refinery and detrimental to the quality of the final petroleum product. Salts such as magnesium chloride, sodium chloride, and calcium chloride are present in most petroleum charges in amounts from about 3 to 200 pounds per thousand barrels of crude oil. These salts are unstable at elevated temperatures. If allowed to remain with the petroleum charge throughout the various stages of the refinery operation the salts will disassociate and the chloride ions will hydrolize to form hydrochloric acid. Hydrochloric acid, as well as organic acids which are present to varying degrees in the petroleum crude contribute to corrosion in the main fractionator unit and other regions of the refinery system where temperatures are elevated, and where water condenses.
Crude oil desalting is a common emulsion breaking method where an emulsion is first intentionally formed. Water is added in an amount of approximately 5% to 10% by volume of crude. The added water is intimately mixed with the crude oil to contact the impurities therein thereby transferring these impurities into the water phase of the emulsion. The emulsion is usually resolved with the assistance of emulsion breaking chemicals, characteristically surfactants, and by the known method of providing an electrical field to polarize the water droplets. Once the emulsion is broken, the water and petroleum media form distinct phases. The water phase is separated from the petroleum phase and subsequently removed from the desalter. The petroleum phase is directed further downstream for processing through the refinery operation.
The water phase, an effluent brine, will contain the salts removed from the petroleum charge. Also present in the desalter effluent brine will be oily solids which remain due to inefficiencies in breaking of the oil - water emulsion and benzene. Because benzene is a volatile organic carcinogen its content in wastewater is restricted by environmental regulations. Accordingly, the effluent brine typically must be treated to reduce the amount of benzene to acceptable levels for disposal.
The combination of dispersed oil, solids, oily solids and benzene in the effluent brine make treatment difficult. The benzene can be removed by solvent extraction with a petroleum distillate oil solvent. However, the dispersed oil, solids and oily solids can interfere with this extraction process. The present inventor discovered that appropriate treatment of the effluent brine, while hot or when it is subsequently reheated can enhance the removal of benzene from the effluent brine.